This invention relates to polymeric fibers and more particularly to filler-enhanced fibers with improved axial, transverse and shear mechanical properties.
Current technology produces aramid and rigid rod polymers with axial stiffness approximately only one-half of the theoretical limits because of misalignment of the underlying molecular morphology in the polymer chains. Furthermore, the orientation of van der Waals interaction planes versus hydrogen bonding planes results in low shear modulus. The molecular morphology misalignment, together with the low shear modulus and orientation of hydrogen bonding planes, also produce poor axial compressive strength of a fiber.
As an example, the prior art dry-jet wet spinning process has been used to produce high performance fibers with highly extended polymer chains as taught in U.S. Pat. No. 3,767,756. It is well known that this process results in chain misorientation at different length scales ranging from paracrystallinity at the nanometer length scale to a pleated structure at the submicron to micron scale. The pleated structure within the fibers is shown in the schematic of FIG. 1 and can be seen in an optical micrograph in Yang, H. H., “Kevlar aramid fiber,” Chichester, U.K.: Wiley (1993). For fibers such as Kevlar® and Twaron® that have high crystallinity and planar hydrogen bond interaction, hydrogen bonding is along the radial direction of the fibers. The misorientation of chains resulting in pleating ranges from 6.8° to 21.0° for different types of Kevlar fibers under different conditions in which the pleated structure contributes to chain misorientation from 5° to 10°. See, Rao, et al., Polymer, 5937 (2001). This amount of misorientation results in a dramatic decrease in the fiber axial modulus from the ideal fiber axial elastic modulus known as its chain modulus. For example, the ideal fiber axial modulus of Kevlar can be as high as 240 GPa (Lee, et al., J. Polym. Sci., Part B: Polym. Phys., 1 (1995)), but the measured Kevlar fiber axial stiffness is only 129.6 GPa, (Kawabata, et al., 9th International Conference on Composite Materials, Spain (1993)).
The shear properties of these high performance fibers are also quite poor because of their highly anisotropic microstructures. The shear modulus and shear strength of Kevlar fiber are 1.8 GPa and 0.18 GPa respectively (Yang, above). The compressive strength along the fiber axial direction is correlated with the fiber shear properties and is therefore very poor. For the same reason, compressive strength along the fiber transverse direction is also poor.